Python3中‘variable: value’语法的含义是什么？

Great question! Let's unpack this syntax—it’s a perfect example of how Vyper repurposes valid Python syntax to fit Ethereum smart contract development needs.

First, in standard Python 3.6+:
This is a valid variable annotation defined in PEP 526. Python lets you use any legal expression as a variable annotation, not just type objects. Here’s what that actually does:

• The line myvariable: somefunction(someglobalvariable) doesn’t assign a value to myvariable. It only stores the result of somefunction(someglobalvariable) in the module or class’s __annotations__ dictionary as metadata.
• For example, run this in standard Python:

  someglobalvariable = 5
  def somefunction(n):
      return n * 3
  
  myvariable: somefunction(someglobalvariable)
  print(__annotations__)  # Output: {'myvariable': 15}
  print(myvariable)       # Throws a NameError—myvariable was never assigned a value!
  

  The annotation is just extra information; it doesn’t affect the variable’s state unless a tool (like a type checker or custom script) reads the __annotations__ dict.

Now, in Vyper:
Vyper is a strict subset of Python, so all its syntax is valid Python—but it gives some features new, blockchain-specific meanings. In this case:

• The line myvariable: somefunction(someglobalvariable) is how you declare and initialize a state variable. Vyper interprets somefunction(someglobalvariable) as the initial value for myvariable, and infers the variable’s type from the function’s return type.
• Unlike standard Python, this line actually creates the state variable and sets its value (which gets written to the Ethereum blockchain when the contract is deployed).
• For context, Vyper requires explicit typing for all variables—under the hood, this is equivalent to something like myvariable: uint256 = somefunction(someglobalvariable) if somefunction returns a uint256; the type is just inferred here for brevity.

The key takeaway: The syntax is 100% valid Python, but its behavior shifts drastically depending on whether you’re running it as standard Python or compiling it as Vyper. Vyper leverages Python’s annotation syntax to create a concise way to define state variables with initial values.

内容的提问来源于stack exchange，提问作者Marco Merlini